The present invention relates to a biomaterial, to the process for its preparation and to its applications, in particular as repair or filling material in the osteoarticular, dental or maxillofacial field.
To consolidate bone defects, methods commonly used are autografting or allografting. Corticospongy autografting is the oldest method: it makes it possible to obtain good quality bone consolidation with perfect immune tolerance and with no risk of transmission of pathogenic agents. However, the bone stock is limited in an individual, especially in children, and the additional surgical act which results therefrom causes risks of complications. As for the allograft, which is removed from subjects in a state of cerebral death, it only exhibits osteoconductive and nonosteoinductive properties since the bone removed is frozen so as to destroy all the components which could trigger an immune reaction of graft rejection by the recipient body or could be a vector of infections. This treatment reduces, in addition, the mechanical properties of bone.
By virtue of the insufficiency of bone grafts, bone replacement materials are currently the essential research route for promoting bone consolidation.
These materials are of considerable public health importance. Indeed, numerous synthetic or metallic materials are used as bone replacement materials in indications as diverse as bone graft or total hip prosthesis. Currently, 60 000 total hip prostheses are implanted each year in France. In 1988, an epidemiological study showed that half of the implants fitted in 16 million American patients were orthopedic implants. These statistics can be extrapolated to the French situation.
Bone consolidation involves various types of cell derived either from the mesenchymal line (preosteoblasts, osteoblasts and osteocytes), or from the hematopoietic line (osteoclasts). The osteoblasts, which are located at the bone surface, are involved in the synthesis of a novel bone matrix. The osteocytes are included in the bone matrix; connected to each other, they form a true intercellular network. The osteoclasts, which make it possible to resorb the bone, are dependent on growth factors. The osteoclastic activity is essential for reconstruction, because it is itself capable of amplifying the activities of synthesis of osteoblasts by means, inter alia, of growth factors. The platelets also have an important role: they release large quantities of growth factors, at the initial stage of bone repair.
Growth factors are a category of polypeptides having properties which regulate numerous parameters of cell life (such as proliferation, differentiation, survival). These factors are secreted by multiple types of cell. The effects of the same factor are determined by the nature of the target cell, the concentration of the factor, the possible simultaneous presence of other factors. The target cells possess membrane receptors for these factors, most often tyrosine kinases. Their stimulation regulates the synthesis or the activity of regulatory proteins. The names of the growth factors are most often taken from the material where they were detected for the first time and are not always representative of their function.
Bone tissue contains a variety of growth and differentiation factors which control bone formation and resorption and which also play an important role in the development, growth and repair of cartilage and bone. These principal factors are EGFs (“Epidermic Growth Factors”), IGFs (“Insulin-like Growth Factors”), FGFs (“Fibroblast Growth Factors”), TGF-β (“Transforming Growth Factors”), PDGFs (“Platelet-Derived Growth Factors”) and BMPs (“Bone Morphogenetic Proteins”).
The BMPs are part of the TGF-β superfamily. Their osteoinductive activity was demonstrated by M. R. Urist in 1965 (Science, 1965, 150, 893-899): demineralized bone was implanted at an ectopic, namely intramuscular, site in rats and gave rise to the formation of cartilage, and then bone. The proteins extracted from the demineralized bone and which are responsible for this bone induction were purified and called Bone Morphogenetic Proteins (BMP). Seven proteins (BMP-1 to 7) were initially cloned using molecular biology techniques (Wozney, J. M. et al., Science, 1988, 242, 1528-1534). It has since been shown that BMP-1 does not have osteoconductive properties. To date, numerous other BMPs have been cloned.
The use of BMPs alone in bone repair involves the injection of large quantities, much higher than those effective in in vitro studies or detected in normal tissues.
Moreover, the systemic effect of these products exhibits risks of diffusion of BMP into the surrounding muscle mass and of local calcification in a non-bone site. In addition, BMPs have very short lives, of the order of a few minutes in free form.
It would therefore be desirable to develop a vector for BMPs as well as for the other growth factors involved in bone repair.
T. R. Gerhart et al. (Clinical orthopaedics and related research, 1993, 293, 317-326 and 1995, 318, 222-230) have proposed mixing human recombinant BMP-2 with inactive bone matrix (1.5 mg of BMP per 3 g of bone matrix) in order to produce bone implants. These implants make it possible to induce reossification of defects 2.5 cm long artificially created in the femurs of sheep. However, the quantity of BMP provided by the implant (1.5 mg) is 25 000 times higher than the quantity of BMP endogenously present in the native bone tissues (˜0.06 μg).
Other types of supports have also been proposed: collagen, hydroxyapatite, gelatin, tricalcic calcium phosphate, calcium sulfate, calcium carbonate, coral, polymers of polylactic and polyglycolic acids, and the like.
All these organic or inorganic, natural or synthetic compounds are not really vectors for BMP. They indeed exhibit affinity which is good to a greater or lesser degree with the growth factor.
In parallel with these studies, it has been shown that, in a manner similar to heparin and to heparan sulfates, dextrans (D) substituted with carboxymethyl (MC), benzylamide (B) and sulfonate (S) groups (compounds called DMCBS) interact with certain growth factors, HBGFs (Heparin Binding Growth Factors), in particular with FGFs and TFG-β. They potentiate the biological effects of these endogenous factors, released at the lesioned site, protecting them against degradations of thermal, acidic or proteolytic origin (F. Blanquaert et al., Bone, 1995, 17, 6, 499-506; A. Meddahi et al., Journal of Biomedical Materials Research, 1996, 31, 293-297; J. Lafont et al., Growth factors, 1998, 16, 23-38; F. Blanquaert et al., Journal of Biomedical Materials Research, 1999, 44, 63-72).
In the field of bone repair, particular CMDBSs, RGTA9 and RGTA11 (compounds comprising respectively 83% or 110% of CM units, 23% or 2.6% of B units and 13% or 36.5% of S units), immobilized in a collagen support, have made it possible to induce bone regeneration (abovementioned articles by F. Blanquaert et al., 1995 and J. Lafont et al., 1996). It has thus been shown that RGTA9 and RGTA11 themselves, without addition of exogenous growth factors, make it possible to stimulate bone reconstruction. F. Blanquaer et al., 1995, explain this property by the fact that RGTA9, which is vectorized in collagen and complexed with the growth factors endogenously present, could constitute a reservoir of said growth factors for their subsequent release. J. Lafont et al. propose using RGTA11 as such, transported in a collagen support, instead of administering growth factors.
In short, F. Blanquaert et al. and J. Lafont et al. therefore propose vectorizing RGTA9 or RGTA11 in a collagen support, which is known to induce a rapid and uncontrolled kinetics of release.
Moreover, the abovementioned article by F. Blanquaert et al. (1999) describes the stimulation of the expression of the osteoblastic phenotype by RGTA9 and RGTA11 placed in contact, in vitro and in soluble form, with growth factors (BMP-2, TGF-β1, FGF-2). It is indicated that this effect results from a capacity of the RGTAs to interact with these growth factors, thus making it possible to promote the wound healing process. It is also indicated that RGTA9 is less effective than RGTA11, both compounds essentially differing in terms of their respective degrees of substitution with sulfonate groups (S), namely 0.13 for RGTA9 and 0.365 for RGTA11. It is evident from these results that a high percentage of S units plays an important role in the properties of the dextran derivative on bone repair.
Thus, the various abovementioned articles (F. Blanquaert et al., 1995 and 1999; A. Meddahi et al., 1996; J. Lafont et al., 1998) show that the DMCBS are capable of trapping these endogenously released growth factors; they play the role of a temporary reservoir for growth factors which are naturally secreted at the lesioned site.
Such combinations between the DMCBSs and the growth factors, which are formed in vivo, do not constitute an effective vector for said growth factors for use in surgery, in particular in spinal, maxillofacial or dental surgery, or any reconstructive surgery. Indeed, their soluble form does not make it possible to control the diffusion of the growth factors at a specific site. In addition, these combinations do not make it possible to restore the geometry of the bone pieces destroyed, in particular of large bone defects, in which the endogenous growth factors are not present in a sufficient quantity to be able to initiate spontaneous consolidation. It is then necessary to add exogenous growth factors, which have to be used in vivo in combination with a suitable releasing system.